The license is Affero GPL, which means you need to open-source your code even if you use it for a service. "Traditional" GPL was effective only while redistributing. That means you would need to go for commercial license whenever you build a service on it. Which in fact is a fair approach for a business model when there is a company behind an open source project. Especially that this time there is no lock-in. You could always come back to Cassandra.